1. Field of the Invention
The present invention relates to an optical transmission sheet (also referred to simply as an optical sheet) provided with means for setting a propagation angle of a propagating light beam in a thickness direction of a planar waveguide (the definition of this propagation angle will be described later). The present invention also relates to an optoelectric apparatus including an optical circuit provided with the optical sheet and an electronic circuit. Additionally, the present invention relates to an optical transmission method of performing multiplexing-transmission of optical signals divided with different or varied propagation angles in the optical sheet, or performing routing of optical signals with desired propagation angles in the optical sheet.
2. Description of the Related Background Art
Recently, high-speed processing, small size, and light weight are strongly required in personal computers, and information processing apparatuses, such as cellular phones and personal digital assistants (PDAs). It can be, however, pointed out that adverse influences of wiring delay increase in electronic circuit substrates as the processing speed goes up. The simplest method for solving such a disadvantage is to reduce the length of electric wires in chips or between chips to be as short as possible. With respect to such a method, a number of proposals have been so far reported since this method can also achieve compact circuit substrates.
Another problem, however, arises as the processing speed increases. This is the problem of electromagnetic interference (EMI) whose meaning is as follows. When electronic devices are disposed close to each other, the density of electric wires increases even though the length of the wires decreases. As a result, when high-speed signals flow though signal wires close to each other, electromagnetically-induced electromagnetic waves interfere with each other and generate noise, thereby causing erroneous transmission of signals.
As a method for preventing the EMI problem, the following scheme is normally taken. Plural electronic circuit substrates are layered to enhance electromagnetic compatibility (EMC) in each layer. This measure, however, has disadvantages in cost and yield.
As another scheme, methods of using line (line-shaped) waveguides or optical wiring are disclosed in Japanese Patent Application Laid-Open Nos. 5(1993)-67770 and 6(1994)-308519, for example. The linear optical wiring is advantageous in that it is inherently EMI-free. In those schemes, linear waveguides with thicknesses from several microns to several tens microns are used as the line-shaped optical wiring. Those methods, however, have disadvantages in that optical alignment between optical elements is difficult to achieve, a fine-process technique is needed to form the optical waveguides, the number of components is large, and fabrication is difficult.
Further, since much information is handled at a high rate in the optical wiring, wavelength multiplexing, space multiplexing, or the like is said to be necessary in the optical wiring. For such purposes, however, the number of components and costs inevitably increase, and hence, their practicability has not yet been attained.
On the other hand, methods of using the planar waveguide (also known as an optical transmission sheet or an optical sheet) as an optical bus are disclosed in Japanese Patent Application Laid-Open Nos. 11(1999)196069 and 11(1999)-355249, for example. In the former, optical pulse signals with different intensity levels are treated in a superposed manner. In the latter, optical pulse signals at different wavelengths are treated in a superposed manner. In those methods, however, input and output of optical signals are performed only at end faces of the waveguide, so flexibility and extensibility of design are restricted in applying those methods to the optoelectric substrate.